1. Field of the Invention
This invention relates to a group of compounds classified in the field of organic chemistry as 3-heteroaryl-3-(diphenylamino)phthalides useful as color formers in pressure-sensitive carbonless duplicating systems, thermal marking systems and hectographic or spirit-reproducing copying systems; to processes for the preparation thereof; and to pressure-sensitive duplicating systems thermal marking systems and hectographic copying systems containing the same.
2. Description of the Prior Art
Several classes of organic compounds of widely diverse structural types are known to be useful as color formers for carbonless duplicating systems. Among the more widely recognized classes, there may be named phenothiazines, for example, 2'-anilino-6'-diethylaminofluoran; phthalides, the class with which this invention is concerned, for example, crystal violet lactone; and various other types of color formers currently employed in commercially accepted carbonless duplicating systems. Typical of the many such systems taught in the prior art are those described in U.S. Pat. Nos. 2,712,507, 2,800,457 and 3,041,289 which issued July 5, 1955, July 23, 1957 and June 26, 1962, respectively. Many of the color formers in the prior art suffer one or more disadvantages such as low tinctorial strength, poor light stability, low resistance to sublimation and low solubility in common organic solvents, the latter disadvantage thus requiring the use of specialized and expensive solvents in order to obtain microencapsulated solutions of sufficient concentration for use in pressure-sensitive copying systems.
The following appear to constitute the most relevant prior art relative to the present invention.
U.S. Pat. No. 3,736,168, issued May 29, 1973 discloses in most pertinent part a series of phthalides stated to be useful as color formers in pressure-sensitive copying paper and having the formula: ##STR1## wherein inter alia X.sub.1 and X.sub.4 are hydrogen or chloro; X.sub.2 and X.sub.3 are hydrogen, chloro, dimethylamino or diethylamino; R.sub.1 and R.sub.3 are hydrogen, methyl or ethyl; R.sub.2 is hydrogen, methyl or ethoxy; R.sub.4 is methyl, ethyl or phenyl; and R.sub.5 is hydrogen or methyl
U.S. Pat. No. 3,491,112, issued Jan. 20, 1970 discloses the following phthalides which are stated to be useful in pressure-sensitive record materials: ##STR2## R.sub.1 and R.sub.2 are alkyl having fewer than five carbon atoms, phenyl or hydrogen; and R.sub.3 and R.sub.4 are alkyl having fewer than five carbon atoms.